In recent years, attention has been paid to wireless power supply, which does not use a power supply cord and a power transmission cable, as a power supply system that supplies power to a battery mounted on a hybrid automobile or an electric automobile. As one of techniques of the wireless power supply, a technique of resonance type is known.
As the power supply system of resonance type, for example, a supply system illustrated in FIG. 15 is proposed (Patent Literature 1). As illustrated in FIG. 15, a power supply system 100 includes a primary coil unit 102 and a secondary coil unit 103. The primary coil unit 102 is installed on the ground or the like of power supply facilities having an AC (alternating current) power supply 101, to supply power from the AC power supply 101 without contact. The secondary coil unit 103 is mounted on a vehicle to receive power from the primary coil unit 102 without contact.
The primary coil unit 102 includes a primary (power supply side) electromagnetic induction coil 104, a primary resonance coil 105, and a primary capacitor C1. The primary electromagnetic induction coil 104 is connected to the AC power supply 101. The primary resonance coil 105 is supplied with power from the primary electromagnetic induction coil 104 by electromagnetic induction. The primary capacitor C1 is connected to the primary resonance coil 105 to adjust a resonant frequency.
The secondary coil unit 103 includes a secondary (power receipt side) resonance coil 106, a secondary electromagnetic induction coil 107, and a secondary capacitor C2. The secondary resonance coil 106 conducts magnetic field resonance with the primary resonance coil 105. The secondary electromagnetic induction coil 107 is supplied with power from the secondary resonance coil 106 by electromagnetic induction and connected to a load 108. The secondary capacitor C2 is connected to the secondary resonance coil 106 to adjust the resonant frequency.
According to the above-described power supply system 100, when power from the AC power supply 101 is supplied to the primary electromagnetic induction coil 104, the power is sent to the primary resonance coil 105 by electromagnetic induction. As a result, magnetic field resonance is caused between the primary resonance coil 105 and the secondary resonance coil 106. Accordingly, wireless transmission of power from the primary resonance coil 105 to the secondary resonance coil 106 is conducted. In addition, the power sent to the secondary resonance coil 106 is sent to the secondary electromagnetic induction coil 107 by electromagnetic induction. The power is supplied to the load 108 connected to the secondary electromagnetic induction coil 107.
When the power supply system 100 is mounted on power supply facilities or a vehicle, however, a variation of a distance between the resonance coils 105 and 106 (hereafter abbreviated to “inter-coil distance”) and position deviations of the resonance coils 105 and 106 occur. Occurrence of the distance variation and position deviations causes impedance mismatching. Consequently, power is reflected, resulting in lowered transmission efficiency.
This will now be described in more detail with reference to FIGS. 16 and 17. In the power supply system 100, impedance adjustment is conducted to make the transmission efficiency best when the inter-coil distance is 200 mm. FIG. 16 is a graph indicating frequency characteristics of an S parameter S21 between the resonance coils 105 and 106 in each of cases where the inter-coil distance is set equal to 100 mm, 200 mm, 300 mm and 400 mm in the power supply system 100 subjected to the impedance adjustment. In the power supply system 100, impedance adjustment is conducted to make the transmission efficiency best when the inter-coil distance is 200 mm. FIG. 17 is a graph indicating the transmission efficiency between the resonance coils 105 and 106 as a function of the inter-coil distance in the power supply system 100 subjected to the impedance adjustment.
If the inter-coil distance becomes larger than 200 mm in the conventional power supply system 100, coupling between the resonance coils 105 and 106 becomes weak accordingly and the S parameter S21 becomes low, resulting in lowered transmission efficiency as illustrated in FIG. 17. If the inter-coil distance becomes smaller than 200 mm, the coupling between the resonance coils 105 and 106 becomes too strong accordingly and bi-resonant characteristics are brought about as illustrated in FIG. 16. As a result, the S parameter S21 at a transmission frequency (a frequency of the AC power supply 101) becomes lower and the transmission efficiency is lowered.
As a countermeasure against the above-described inter-coil distance and position deviation, it is usually considered to provide a matching circuit in the primary coil unit 102 or the secondary coil unit 103 (or in both the primary coil unit 102 and the secondary coil unit 103 in some cases) to conduct impedance matching. A variable capacitor is provided in the matching circuit. Impedance matching can be executed by changing a capacitance.
In a case where the frequency of the transmission frequency is in a kHz region, however, a capacitor having a large capacitance is needed. It is inevitable to use a film capacitor or a ceramic capacitor. However, there is a problem that it is difficult to make the film capacitor or a ceramic capacitor variable.